Venous Function

Function of the venous system Function of the venous system Definitions Definitions Mean circulatory filling pressure Mean circulatory filling pressure Two compartment model Two compartment model Dynamic methods of assessing volume status Dynamic methods of assessing volume status

Main Points The venous system functions to maintain filling of the heart. The venous system functions to maintain filling of the heart. The main driving force for venous return is MCFP. The main driving force for venous return is MCFP. The splanchnic vascular bed is the reservoir for venous return. The splanchnic vascular bed is the reservoir for venous return. CVP is useless for volume status unless it is at the extremes. Dynamic measures for fluid responsiveness is informative. CVP is useless for volume status unless it is at the extremes. Dynamic measures for fluid responsiveness is informative.

Definitions Venous capacity Venous capacity Blood volume contained in a vein at a specific distending pressure. Blood volume contained in a vein at a specific distending pressure. Venous capacitance Venous capacitance The relationship between contained volume and distending pressure in a vein. The relationship between contained volume and distending pressure in a vein. Venous compliance Venous compliance Change in volume of blood associated with a change in distending pressure. Change in volume of blood associated with a change in distending pressure.

Unstressed volume Unstressed volume A volume of blood in a vein at a transmural pressure = 0. A volume of blood in a vein at a transmural pressure = 0. Stressed volume Stressed volume The volume of blood in a vein above a zero transmural pressure. The volume of blood in a vein above a zero transmural pressure. The sum of stressed and unstressed volume is the total volume of the system. The sum of stressed and unstressed volume is the total volume of the system.

Pressure Volume 0 P1 V1 Unstressed Stressed Vu Capacity

Stressed volume determines the MCFP and affects venous return and cardiac output. Stressed volume determines the MCFP and affects venous return and cardiac output. Unstressed volume is a reserve that can be mobilized when needed. Unstressed volume is a reserve that can be mobilized when needed. It is helpful to think of the volumes as a tub. It is helpful to think of the volumes as a tub.

Stressed volume Unstressed volume Arterial flow Venous resistance CVP

Function of the Venous System To return blood to the heart and serve as capacitance to maintain filling. To return blood to the heart and serve as capacitance to maintain filling. Veins contain 70% of the blood volume and are 30 times more compliant than arteries. Veins contain 70% of the blood volume and are 30 times more compliant than arteries. Thus they are a reservoir that can easily and immediately change volume to maintain filling pressure in the right heart. Thus they are a reservoir that can easily and immediately change volume to maintain filling pressure in the right heart. The splanchnic veins contain 20% of the total blood volume. The splanchnic veins contain 20% of the total blood volume. These are heavily populated with alpha1 and 2 receptors. These are heavily populated with alpha1 and 2 receptors.

Mean Circulatory Filling Pressure If you stop the heart, flow through the capillaries continues for a brief time as the low compliant/high pressure arteries decompress into the high compliant/low pressure veins. If you stop the heart, flow through the capillaries continues for a brief time as the low compliant/high pressure arteries decompress into the high compliant/low pressure veins. Once the pressure equalizes throughout the entire system, the MCFP can be measured. Once the pressure equalizes throughout the entire system, the MCFP can be measured.

Mean Circulatory Filling Pressure Flow to the heart is determined by the gradient between the central and peripheral venous pressure. Flow to the heart is determined by the gradient between the central and peripheral venous pressure. The driving force for venous return (VR) is: The driving force for venous return (VR) is: (MCFP-CVP)/Venous resistance (MCFP-CVP)/Venous resistance CO is determined entirely by VR as the heart can’t pump more blood than it receives. CO is determined entirely by VR as the heart can’t pump more blood than it receives. VR can go up by increasing MCFP or decreasing CVP (resistance is relatively small). VR can go up by increasing MCFP or decreasing CVP (resistance is relatively small).

MCFP is determined by stressed volume and is normally around 7 – 12 mmHg while CVP is 2-3 mmHg. MCFP is determined by stressed volume and is normally around 7 – 12 mmHg while CVP is 2-3 mmHg. So why does an increase in CVP (by bolus) increase CO in a normal heart? So why does an increase in CVP (by bolus) increase CO in a normal heart? The sudden increase in preload would increase SV temporarily but fall once the volume redistributes to the venous system. The sudden increase in preload would increase SV temporarily but fall once the volume redistributes to the venous system. The stressed volume increases and increases the MCFP greater than CVP. The stressed volume increases and increases the MCFP greater than CVP. The pressure gradient is thus increased and so VR goes up. The pressure gradient is thus increased and so VR goes up. Increased VR = increased CO. Increased VR = increased CO.

Pressure Volume 0 P1 V1 Unstressed Stressed Vu Effect of Fluid Bolus

While venous return can be increased by a fluid bolus which increases stressed volume which increases MCFP (think increasing the amount of fluid in the tub), it can also be increased by venoconstriction. While venous return can be increased by a fluid bolus which increases stressed volume which increases MCFP (think increasing the amount of fluid in the tub), it can also be increased by venoconstriction. This decreases venous capacity (not compliance) which in turn decreases unstressed volume to the benefit of the stressed volume. This decreases venous capacity (not compliance) which in turn decreases unstressed volume to the benefit of the stressed volume. Think moving the outlet hole down. Think moving the outlet hole down.

Stressed volume Unstressed volume Arterial flow Venous resistance CVP

Pressure Volume 0 P1 V1 Unstressed Stressed Vu Effect of Venoconstriction

Two Compartment Model of the Venous System It is helpful to think of the venous system as two connected compartments. It is helpful to think of the venous system as two connected compartments. The splanchnic system is very compliant and slow flow while the non-splanchnic system is noncompliant and fast flow. The splanchnic system is very compliant and slow flow while the non-splanchnic system is noncompliant and fast flow. An increase in resistance in the arteries feeding the splanchnic veins decreases flow and shifts blood into the system circulation. An increase in resistance in the arteries feeding the splanchnic veins decreases flow and shifts blood into the system circulation. A decrease in resistance causes blood pooling in the veins. A decrease in resistance causes blood pooling in the veins.

Dynamic methods of assessing volume status I think it goes without saying that the CVP is a less useful measure of volume status (fluid responsiveness) because of the many factors that influence it. I think it goes without saying that the CVP is a less useful measure of volume status (fluid responsiveness) because of the many factors that influence it. Abdominal pressure Abdominal pressure Pump function Pump function Pericardial pressure Pericardial pressure Thoracic pressure Thoracic pressure Dynamic methods are much more useful Dynamic methods are much more useful

On PPV, inspiration causes increased LVSV because of compression of pulmonary veins, decreased afterload and decreased RV volume from pulmonary compression. On PPV, inspiration causes increased LVSV because of compression of pulmonary veins, decreased afterload and decreased RV volume from pulmonary compression. The increased thoracic pressure at end inflation decreases the gradient for venous return at in a few beats causes a decreased LVSV. The increased thoracic pressure at end inflation decreases the gradient for venous return at in a few beats causes a decreased LVSV. This variation is exacerbated by hypovolemia. This variation is exacerbated by hypovolemia. Variation greater than 12 mmHg better reflects preload inadequacy than CVP. Variation greater than 12 mmHg better reflects preload inadequacy than CVP.

How does that work? Hypovolemia causes a fall in the total volume in the system. Hypovolemia causes a fall in the total volume in the system. The fall in capacity is partly compensated by an immediate reflex venoconstriction. The fall in capacity is partly compensated by an immediate reflex venoconstriction. MCFP initially is preserved to maintain venous return. MCFP initially is preserved to maintain venous return.

Pressure Volume 0 P1 V1 Unstressed Stressed Vu Venoconstriction in Response to a Fall in Total Volume

Once the unstressed volume is completely mobilized into the stressed volume, further fall in the total body volume results in a fall in MCFP and therefore, venous return. Once the unstressed volume is completely mobilized into the stressed volume, further fall in the total body volume results in a fall in MCFP and therefore, venous return.

Pressure Volume 0 P1 V1 Stressed Unstressed Volume Exhausted, Further Fall in Volume P2 V2

Recall that venous return is: Recall that venous return is: (MCFP-CVP)/Venous resistance (MCFP-CVP)/Venous resistance When the MCFP falls and the venous resistance rises, the normal variation in CVP causes a greater variation in venous return which translate into a greater variation in cardiac output/blood pressure. When the MCFP falls and the venous resistance rises, the normal variation in CVP causes a greater variation in venous return which translate into a greater variation in cardiac output/blood pressure. Hence why dynamic changes are more reflective of volume status. Hence why dynamic changes are more reflective of volume status. CVP normally varies and subject to external influences. CVP normally varies and subject to external influences. Dynamic changes allows us a look into the status of the stressed and unstressed volumes. Dynamic changes allows us a look into the status of the stressed and unstressed volumes.

Main Points The venous system functions to maintain filling of the heart. The venous system functions to maintain filling of the heart. The main driving force for venous return is MCFP. The main driving force for venous return is MCFP. The splanchnic vascular bed is the reservoir for venous return. The splanchnic vascular bed is the reservoir for venous return. CVP is useless for volume status unless it is at the extremes. Dynamic measures for fluid responsiveness is informative. CVP is useless for volume status unless it is at the extremes. Dynamic measures for fluid responsiveness is informative.

Function of the venous system Function of the venous system Definitions Definitions Mean circulatory filling pressure Mean circulatory filling pressure Two compartment model Two compartment model Dynamic methods of assessing volume status Dynamic methods of assessing volume status